MPEG encoded video is a worldwide format for digital video storage and retrieval. It uses motion compensated prediction for information reduction thereby generating inter-coded pictures which are predicted from other previously decoded pictures. Such previously generated pictures may be intra-coded if they can be decoded on their own, or may be inter-coded. For providing some random access into an encoded video bitstream, a number of intra- and inter-encoded pictures are in each case arranged as a group of pictures (GOP). A GOP includes at least one intra-coded picture to start decoding with and a varying number of other intra- or mostly inter-coded pictures. The inter-coded pictures normally also include bi-directionally predicted pictures. The GOP structure is chosen such that an optimisation for coding efficiency is achieved while keeping a feature of seamless entrance within the video bitstream, i.e. there will be no losses in the video quality when playback is started with any GOP.
However, this scenario changes if video trickplay is requested. This is true in particular for decoding in a backward play mode. Playing in backward direction (i.e. in a trickplay mode) means displaying the pictures of a GOP in reverse order and across GOP boarders. That is a requirement the GOP structure is not constructed for when motion compensated prediction was used in the (forward) encoding.
Two different techniques for backward mode decoding are known. The first one in general performs temporal sub-sampling of the pictures. The bi-directionally predicted B-pictures are skipped due to missing frame memory capacity, and for larger GOP lengths even the P-pictures at the end of the GOPs are not decoded or are left out during reverse playback. Both sub-techniques are resulting in a jerky video movement and thus picture quality degradation.
The other prior art technique is a very-slow backward mode (single-step mode) in which each picture of a GOP is decoded and presented in reverse order. This is achieved by repeatedly decoding a current GOP from its beginning until the next target picture in reverse order is reached. Such processing is much slower than the original speed and thus generates some kind of slide show effect.